1. Field of the Invention
This invention relates to unique abrasive compositions, and, more particularly, abrasive compositions which are slurries of water-insoluble abrasive polishing agents in combination with humectant, and a unique method for making them.
2. Description of the Related Art
An abrasive substance has been included in conventional dentifrice compositions in order to remove various deposits, including pellicle film, from the surface of teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments which impart an unsightly appearance to the teeth. While cleaning is important, the abrasive should not be so aggressive so as to damage the teeth. Ideally, an effective dentifrice abrasive material maximizes pellicle film removal while causing minimal abrasion and damage to the hard tooth tissues. Consequently, among other things, the performance of the dentifrice is highly sensitive to the abrasive polishing agent ingredient. Conventionally, the abrasive polishing material has been introduced in flowable dry powder form to dentifrice compositions, or via re-dispersions of flowable dry powder forms of the polishing agent prepared before or at the time of formulating the dentifrice.
A number of water insoluble, abrasive polishing agents have been used or described for dentifrice compositions. These abrasive polishing agents include natural and synthetic abrasive particulate materials. The generally known synthetic abrasive polishing agents include amorphous precipitated silicas and silica gels and precipitated calcium carbonate (PCC). Other abrasive polishing agents for dentifrices have included chalk, magnesium carbonate, dicalcium phosphate and its dihydrate forms, calcium pyrophosphate, zirconium silicate, potassium metaphosphate, magnesium orthophosphate, tricalcium phosphate, and the like.
Synthetically-produced precipitated silicas, in particular, have been used as abrasive components in dentifrice formulations due to their cleaning ability, relative safeness, and compatibility with typical dentifrice ingredients, such as humectants, thickening agents, flavoring agents, anti-caries agents, and so forth. As known, synthetic precipitated silicas generally are produced by the de-stabilization and precipitation of amorphous silica from soluble alkaline silicate by the addition of a mineral acid and/or acid gases under conditions in which primary particles initially formed tend to associate with each other to form a plurality of agglomerates (i.e., discrete clusters of primary particles), but without aggregation into a three-dimensional gel structure. The resulting precipitate is separated from the aqueous fraction of the reaction mixture by filtering, washing, and drying procedures, and then the dried product is mechanically comminuted in order to provide a suitable particle size and size distribution.
The silica drying procedures are conventionally accomplished using spray drying, nozzle drying (e.g., tower or fountain), flash drying, rotary wheel drying, oven/fluid bed drying, and the like, which often require considerable expenditures for equipment and operating costs. A similar issue is associated with other synthetically derived polishing agents, such as PCC.
Additionally, conventional abrasive polishing agents intended for dentifrices have required comminution in order to reduce the particle size of the dewatered precipitated silica product down to a size that does not feel gritty in the mouth of a dentifrice user, while, on the other hand, not being so small as to lack sufficient polishing action. That is, in conventional practice, the average particle size of the silica in the reactor formed by acidulation of a metal silicate is too large for dentifrice applications and the like. To comminute dry silica particulates, grinding and milling equipment have been used, such as a hammer or a pendulum mill used in one or multiple passes, and fine grinding has been performed, for example, by fluid energy or air-jet mill. These additional dry comminution operations entail added cost and process time. Moreover, conventional dry grinding and milling equipment and methods tends to introduce impurities into the silica which can diminish the brightness, i.e., cause xe2x80x9cgrayingxe2x80x9d, of the formulation which ultimately incorporates the ground dry silica.
U.S. Pat. No. 3,506,757 to Salzmann describes liquid dentifrices comprising particulate abrasive materials, stably suspended in an aqueous liquid vehicle with the aid of a polysaccharide gum as suspending agent. Similarly, PCT published application no. WO 97/46485 describes silica having a median particle size generally around 12 to 30 xcexcm provided in the form of a suspension, which can be stabilized using a hydrocolloid, particularly naming polysaccharides such as xanthan gum, guar gum, and water-soluble cellulose ethers. U.S. Pat. No. 5,310,543 describes liquid dentifrices containing particulate siliceous abrasive cleaning agents stably suspended in a liquid medium with the aid of a polysaccharide gum and using a liquid medium specified as being substantially free from polyol-type humectants in order to obtain satisfactory rheological properties.
Among other things, Theologically stable liquid abrasive compositions containing appropriately sized abrasive particles would be desirable that could be prepared as part of a continuous process flow and without the need for costly drying and dry milling/comminuting post-treatments.
The above and other objectives, advantages and benefits are achieved by the present invention directed to unique abrasive compositions comprised of water-insoluble abrasive polishing agents suspended in a liquid medium in combination with humectant. The inventive abrasive compositions are pumpable, Theologically stable and re-agglomeration resistant, even during and after transport and/or storage before end-use, such as incorporation into dentifrice formulations or other oral cleaning compositions. The inventive abrasive compositions do not require temporary stabilizers comprised of inorganic suspending agents (e.g., fumed silicas) or organic binders (e.g., polysaccharides) in order to possess adequate resistance to settling. If a binder material is desirable in a dentifrice composition or other oral cleaning composition ultimately incorporating the inventive abrasive composition, its addition can be deferred until that time. Indeed, experimental studies reported herein indicate a destabilizing effect of a binder where it was incorporated into a relatively high solids, aqueous silica slurry containing humectant.
The invention also concerns a method for making such abrasive compositions as a continuous, integrated process without any need for drying procedures prior to end-use of the abrasive composition. Capital and energy costs are saved by eliminating silica drying operations. Further, the comminution requirements for the abrasive particle content of the inventive slurries are reduced, if not completely eliminated in some cases, by effectively performing comminution concurrent with synthesis of the abrasive material in a common reactor system in the practice of this invention. This is accomplished in this invention by use of a reactor system involving in situ high shear mixing of a portion of an acidulation reaction mixture. In one preferred implementation, a portion of the acidulation reaction mixture is continuously pumped through through a high shear mixer located in a recirculation loop provided on the reactor. In this manner, the water-insoluble abrasive particulate product of the reaction can be provided with the requisite particle size suited for oral cleaning compositions to eliminate the need for drying and comminuting dry silica solids after filtration. While not desiring to be bound to any particular theory at this time, it is postulated that the rheological properties of the inventive abrasive compositions are superior, at least in part, due the avoidance of agglomeration of particles otherwise incurred by the silica particles during conventional silica particle drying procedures. By contrast, the abrasive composition products made by the inventive method contain silica particles that retain or essentially retain the original structure and chemistry of the reactor silica and avoid changes in surface hydroxyl group type and density. In this regard, the undried abrasive particles used in slurries made according to this invention show improved TAPPI brightness values as compared to abrasive particles derived from processing involving abrasive particle drying and dry comminution. xe2x80x9cDryingxe2x80x9d of the silica particles, for purposes herein, means silica particles have been dehydrated to an extent that a generally dry flowable powder results as the water content is reduced below about 10 wt %. Thus, xe2x80x9cdriedxe2x80x9d or xe2x80x9cdryxe2x80x9d abrasive particles have been subjected to such drying, as defined above. Also, xe2x80x9cdry comminutionxe2x80x9d, means milling or other mechanical attrition performed on dried abrasive particles. The abrasive particles used in the inventive slurries are not dried nor dry comminuted.
In a preferred, optional embodiment of this invention, a small amount of an anti-microbial preservative can be added concurrent with the introduction and mixture of the humectant with the wet cake.
The inventive abrasive compositions are ready-to-use additives in the preparation of oral cleaning compositions, such as dentifrices, toothpastes, and the like, particularly suited as a raw material in a continuous toothpaste making process.